U.S. Pat. No. 8,642,503 B1 discloses a superhydrophobic and superoleophilic composite including a porous material and a surface layer. The porous material includes a framework that contains a plurality of skeletons connected with each other and a plurality of interconnecting pores separated by the skeletons (i.e., sponge-like structure). The porous material is made of lignocellulose, cellulose, foamed polymer, polyether, polyvinyl alcohol, melamine, polyurethane, or polyester. The surface layer is coated on surfaces of the skeletons and includes an adhesion medium and a plurality of graphene sheets stuck to the surfaces of the skeletons by the adhesion medium. The composite has a water contact angle of 150-170 degrees (superhydrophobicity), and an oil contact angle of 0-10 degrees (superoleophilicity). Therefore, the composite can be used to absorb oil or organic pollutants in water, thereby achieving oil-water separation.
However, due to the superhydrophobicity of the composite, water cannot flow therethrough. Since oil has a specific gravity smaller than that of water, oil would float on water (i.e., water forms a lower layer and oil forms an upper layer). During oil-water separation, the water would be restrained from passing through the composite and massively accumulate above the composite so that the oil may be prevented from effectively contacting with the composite so as to greatly reduce the separation efficiency. Moreover, the accumulated water might destroy the structure of the composite or a device equipped with the composite.